1. Field of the Invention
This invention relates generally to an EPDM rubber composition for vibration isolators and such dynamic articles using the composition, more particularly to a rubber composition comprising high-molecular-weight gas-phase metallocene EPDM, ground mica, and silicone-modified EPDM.
2. Description of the Prior Art
Vibrations occur in vehicles, rotating or reciprocating machinery, and the like. Vibrations may be isolated, or absorbed or damped by appropriate placement of rubber members. The term “vibration isolation” will be used herein in its most general sense and is intended to encompass isolation, absorption, and damping of any of various modes of vibration, shock, or the like. “Vibration isolator” will be used herein for any kind of device for reducing, damping or absorbing vibration, shock, or associated noise or harshness or for isolating or preventing the transmission thereof from its source to another point in a dynamic system or to a user or observer.
Rubber for dynamic articles such as isolators and other dynamic applications of elastomers, must generally meet demanding physical requirements, such as tear strength, heat resistance, low compression set, flex fatigue, etc. Some of these properties tend to be in opposition, making improved rubber compounds difficult to achieve. For example, increased crosslink density of cured rubber generally helps reduce compression set, but also results in reduced tear strength. Natural rubber has been a preferred elastomer for isolators because of its high strength, but it is lacking in heat resistance.
Ethylene-alpha-olefin elastomers such as ethylene-propylene copolymers and ethylene-propylene-diene terpolymers (“EPDM”) are general purpose synthetic elastomers known for good heat resistance, economical, easy to process, but low tear strength and poor dynamic fatigue resistance when compounded for low compression set and heat resistance, for example, by the use of peroxide cure systems. Use of EPDM in dynamic applications has been rather limited, especially for peroxide-cured EPDM. Peroxide-cured compositions suitable for belts are disclosed by U.S. Pat. No. 5,610,217. Peroxide-cured compositions suitable for rubber-metal bonded vibration isolators such as crank dampers are disclosed in U.S. Pat. Nos. 7,078,104 and 7,166,678. Sulfur-cured compositions suitable for dynamic applications as a replacement for natural rubber parts are disclosed in U.S. Pat. No. 6,693,145.
Silicone-modified EPDM is disclosed for example in U.S. Pat. No. 6,380,312 for use in oil resistant thermoplastic elastomer blends. Elastomeric blends of EPDM with silicone-modified EPDM are disclosed in U.S. Pat. No. 5,985,970 for the purpose of producing a tack-free surface on crosslinked EPDM products.
Mica is known for general use as a filler, especially in plastics, for specific use as non-conducting or insulating filler in elastomers or plastics, and for use as a dusting agent to prevent sticking of uncured rubber stocks, and the like. Mica is also known to adversely affect the fatigue life of carbon-black-filled or silica-filled elastomeric compounds.